Radial shaft seals are used in the vehicle field to protect wheel bearings by providing a barrier between a rotatable wheel hub and a stationary wheel spindle or shaft. Such wheel bearings require continuous lubrication to reduce friction during operation.
The Scotseal® Plus XL wheel shaft seal, available from SKF USA, Inc. for several years, has been successfully employed as a radial fluid seal for heavy duty applications, such as trucks and buses. In addition to retaining lubricant around the wheel bearings, it also effectively prevents contamination, such as dirt and water, from reaching the wheel bearings.
An example of the known Scotseal® Plus XL is shown in FIG. 1, and it includes two basic annular components, the outer shell or case 1 and the inner shell or sleeve 20.
The outer shell 1 includes a substantially L-shaped metal reinforcement 2 and an elastomeric sealing element 3 made of a hydrogenated nitrile butadiene rubber (HNBR), preferably a rubber satisfying the ASTM D2000 callout M3DH810 A26 B16 EO16 EO36. The elastomeric sealing element 3 includes a primary seal lip 4, a radial dust/dirt lip 5, an annular dust/dirt lip 6 and an outer bump lip 7, all of which are elastic and contact a surface of the metal sleeve 20 during operation. The primary seal lip 4 serves to retain the lubricant around the wheel bearings (not illustrated, but located to the right of the lip 4 in FIG. 1) and the other three lips 5, 6 and 7 serve to prevent contamination from entering into the lubrication chamber (sealed region). A plurality of annular beads 8 are defined on the outside diameter (outside circumferential surface) of the elastomeric sealing element 3 for sealingly abutting against an adjacent surface of the wheel hub.
A garter (annular coiled wire) spring 9 serves to radially-inwardly bias or load the primary seal lip 4 into sealing contact with the metal sleeve 20.
The primary seal lip 4 has a Wave® seal lip construction, which reduces friction and thus heat generation while also serving to pump the lubricant back towards the wheel bearings and push dirt away from the lubrication chamber. For this purpose, the primary seal lip is a smooth lip, bi-rotational, hydrodynamic radial lip seal formed in a sinusoidal or wave pattern. This design also reduces shaft wear and increases service life, while not losing pumping power as it wears.
While the Scotseal® Plus XL performs in a satisfactory manner, new vehicle designs have become more aerodynamic, and this has decreased the volume of ambient air that flows past the seal during use (on a truck or a bus, for example). In addition, it is becoming increasingly common to use less lubricant inside the volume enclosed by the seal, and this leaves less lubricant available to lubricate the interface between the seal and the sleeve. The combination of reduced cooling air and reduced lubrication causes seals to operate at higher temperatures than in the past, and these higher operating temperatures degrade the HBNR seal material from more quickly than when the seals operated at lower temperatures.